The present invention relates to an optical information recording/reproducing optical system and an optical information recording/reproducing apparatus for recording information to and/or reproducing information from an optical disc based on a predetermined standard by irradiating an information recording layer of the optical disc with a laser beam, and particularly to an optical information recording/reproducing optical system and an optical information recording/reproducing apparatus suitable for information recording/reproducing for a high-recording density optical disc, such as BD (Blu-ray Disc).
There exist various standards of optical discs, such as CD (Compact Disc) and DVD (Digital Versatile Disc), differing in recording density, protective layer thickness, etc. Meanwhile, high-recording density optical discs (e.g., BD), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. Incidentally, in this specification, the “optical information recording/reproducing apparatuses” include apparatuses for both information reproducing and information recording, apparatuses exclusively for information reproducing, and apparatuses exclusively for information recording, and the same thing can be said for the “optical information recording/reproducing optical systems”. In the following, the “optical information recording/reproducing optical system” is frequently referred to as an optical system of an optical pick-up.
For information recording and reproducing for the high-recording density optical disc (e.g., BD), blue laser light having the wavelength of approximately 400 nm (which is also called violet laser light or bluish-violet laser light) is used. Since such laser light close to the ultraviolet region has a high photon energy, the laser light may have chemical effects on an optical element, such as a resign lens, configuring an optical system of an optical pick-up. One of the chemical effects is a white turbidity phenomenon which is caused in a resign base material of an optical element when the optical element is irradiated with the blue laser light having an intensity of a certain value or more for a long time under a high temperature condition. When the white turbidity phenomenon occurs in the base material of the optical element, transmissivity of the optical element decreases, and strong scattering light is caused. As a result, the optical performance of the optical system of an optical pick-up deteriorates. The white turbidity phenomenon frequently occurs when resin having a relatively large degree of absorption at the wavelength of blue laser is used as material of the optical element.
Various analyses for preventing occurrence of the white turbidity phenomenon are now being made. For example, Japanese Patent Provisional Publication No. 2005-266780A discloses a technique where an addition agent, such as a soft polymer or an alcoholic compound, is added to the resign forming the base material so that the property of the resin is altered to become hard to cause the white turbidity phenomenon.
However, if a new resin material is employed, it becomes necessary to conduct various types of appropriateness reevaluations concerning the resin base material, such as a processing condition of the base material, various optical films to be coated on the base material and an adhesive agent, in addition to conducting evaluation of the new resin itself. Therefore, use of a new resin material involves a high degree of risk and the cost increase. It is desired to prevent occurrence of the white turbidity by a technique other than altering the resin base material.